1. Field of the Invention
The present invention relates to controlling a reticle stage during exposure.
2. Background Art
Historically, in lithographic tools a mounting side and a patterned side of a reticle are one and the same, establishing a reticle focal plane at a plane of a reticle stage platen. Thus, knowledge of stage position in six degrees-of-freedom (DOF) resulted in knowledge of the reticle patterned surface position in six DOF. The six DOF are X, Y, Z, Rx, Ry, and Rz, as shown in FIG. 1. However, mounting (or clamping) of an extreme ultra violet (EUV) reticle will almost certainly be to a back surface of the reticle (e.g., opposite from the patterned surface). Backside clamping results in a reticle focal plane position relative to the reticle stage that is a function of reticle flatness, reticle thickness, and reticle thickness variation. Thus, in contrast to deep ultra violet (DUV) systems, knowledge of the reticle stage position does not resolve where the pattern of the reticle is located in all six DOF. The out-of-plane DOF (Z, Rx, and Ry) cannot be easily determined due to the thickness variation of the reticle. The position of the patterned side (opposite to the clamped side) of the reticle needs to be known accurately in all six DOF.
In almost all steppers and scanners three in-plane DOF (X, Y, and Rz) are determined from typical stage metrology schemes using interferometers. However, three out-of-plane DOF (Z, Ry, and Rx) are more difficult to measure. As discussed above, in an EUV tool, Z, Rx, and Ry have to be known with much higher accuracy than in previous lithography tools. The accuracy requirement stems from the need to position the pattern on the reticle at a focal plane related to optics of the lithography tool. Also, in some cases, optics are not telecentric at the reticle focal plane, which increases the need for accuratley determining the reticle position on the reticle stage to within six DOF. At the same time, it is critical to accurately maintain focus on the pattern on the reticle even though the reticle is not perfectly flat. Therefore, measuring the Z position and the out of plane tilts (Rx and Ry) of the patterned side of the reticle in the EUV tool requires tight accuracy.
Therefore, what is needed is a measuring system and method that can easily calibrate or correlate a reticle focal plane (for a backside clamped reticle) to a reticle stage to allow tracking of a patterned surface of a reticle's position in six DOF using reasonably conventional stage metrology methods. A measuring system and method is also needed that maps a reticle surface to surfaces on a reticle stage, which allows feedback for stage position to be based on surfaces on the stage instead of surfaces on the reticle surface.